3.16 Guarded Evaluation: when and unless

The Racket Reference

3 Syntactic Forms

3.1 Modules: module, module*, ...

3.2 Importing and Exporting: require and provide

3.3 Literals: quote and #%datum

3.4 Expression Wrapper: #%expression

3.5 Variable References and #%top

3.6 Locations: #%variable-reference

3.7 Procedure Applications and #%app

3.8 Procedure Expressions: lambda and case-lambda

3.9 Local Binding: let, let*, letrec, ...

3.10 Local Definitions: local

3.11 Constructing Graphs: shared

3.12 Conditionals: if, cond, and, and or

3.13 Dispatch: case

3.14 Definitions: define, define-syntax, ...

3.15 Sequencing: begin, begin0, and begin-for-syntax

3.16 Guarded Evaluation: when and unless

3.17 Assignment: set! and set!-values

3.18 Iterations and Comprehensions: for, for/list, ...

3.19 Continuation Marks: with-continuation-mark

3.20 Quasiquoting: quasiquote, unquote, and unquote-splicing

3.21 Syntax Quoting: quote-syntax

3.22 Interaction Wrapper: #%top-interaction

3.23 Blocks: block

3.24 Internal-Definition Limiting: #%stratified-body

3.25 Performance Hints: begin-encourage-inline

3.26 Importing Modules Lazily: lazy-require

On this page:

top ← prev up next →

3.16Guarded Evaluation: when and unless 🔗 i

+Effects If...: when and unless in The Racket Guide introduces when and unless .

syntax
( when test-exprbody...+)

Evaluates test-expr. If the result is #f, then the result of the when expression is #<void>. Otherwise, the bodys are evaluated, and the last body is in tail position with respect to the when form.

Examples:

> (when (positive? -5)
(display "hi"))

> (when (positive? 5)
(display "hi")
(display " there"))

hi there

syntax
( unless test-exprbody...+)

Equivalent to (when (not test-expr)body...+ ).

Examples:

> (unless (positive? 5)
(display "hi"))

> (unless (positive? -5)
(display "hi")
(display " there"))

hi there

top ← prev up next →